The R-T-B based permanent magnet (R is rare earth element(s), T is Fe or Fe with part of which is replaced with Co, and B is boron) having the tetragonal compound R2T14B as the main phase is known to have excellent magnetic properties and has been a representative permanent magnet with high performance since the invention in 1982 (Patent document 1: JPS59-46008A).
The R-T-B based magnets with the rare earth element(s) R being consisted of Nd, Pr, Dy, Ho and Tb have a large anisotropy magnetic field Ha and are preferably used as permanent magnet materials. Among them, the Nd—Fe—B based magnet having Nd as the rare earth element R is widely used because it has a good balance among saturation magnetization Is, Curie temperature Tc and anisotropy magnetic field Ha, and is superior in resource abundance and corrosion resistance than R-T-B based magnets using other rare earth elements as R.
As a rotating machine widely used in consumer, industry, and transportation equipment, permanent-magnet synchronous rotating machines tend to be used extensively in terms of saving energy and energy density in recent years.
In a permanent-magnet synchronous motor, the surface magnet type rotating machine, which has a permanent magnet adhered on the surface of the rotor, has the advantage of being capable of effectively utilizing the magnetism possessed by the permanent magnet on one hand. And on the another hand, it has a problem that the permanent magnet adhered on the motor may be peeled off due to a centrifugal force under a high rotating speed.